The use of composite materials has increased in recent years. Composites are very attractive because they offer unique properties and the structure of the composites as well as laminates of the composites can be tailored to meet specific performance requirements. Composites are based on the controlled distribution of reinforcement, which may consist of one or more materials, in a continuous phase of a matrix material. Orientation of the reinforcement affects the strength of composite and can produce high unidirectional strength. To acquire optimum overall strength, layers of composite materials are often laminated such that the orientation of the reinforcement elements of the several layers run in different directions to provide optimum strength in more than one direction.
An initial preparation of a laminate is to prepare a composite sheet. The composite sheet is formed by arranging the reinforcement material in the desired manner and cover the reinforcement with the matrix system. This is then typically rolled or otherwise handled to spread the reinforcement evenly and to produce a uniform thickness for the sheet. This creates an impregnated system which is then partially cured to fix the geometry while allowing enough shape relaxation (drape) and adherence (tack) to permit complex shapes to be built up from the composite sheets thus formed. These composite sheets are commonly called "prepreg" in the industry. A prepreg can also be formed from a woven cloth of the reinforcing material which is then impregnated with resin. When these prepreg sheets are not used immediately to make a desired structure, the surfaces of the sheet are covered with a protective backing to protect the part ally cured sheet.
In the typical fabrication of complex parts, multiple sheets of prepreg are overlayed in the desired orientation. This process is called layup. After layup, the structure formed is cured, thereby bonding the sheets of composite in a laminate. In order to achieve proper bonding of the prepreg composite sheets together to form the laminate, it is important that the surfaces of the prepreg be free from contamination which would interfere with the curing of the laminated structure. Because the prepregs are usually made in a different processing step, the partially cured prepreg material is generally covered with a protective sheet of material to prevent contamination and damage to the prepreg. These coatings are referred to as protective backings. Typical backings used with prepregs have been plastic films such as polyethylene, mylar, and tedlar. These protective backings are later removed prior to the layup procedure. After a part is laid up in the desired shape, the laminate of prepreg material is then subject to final cure which bonds all the prepreg sheets into the single laminated part. In the event that backing material is inadvertently left on a prepreg sheet during layup, the layers of prepreg adjacent to that backing will not properly cure and thus not bond completely resulting in a weakened area of the laminated part. Thus, it is important to assure that all backing material is removed and that the layup does not contain any backing. The backings currently used can only be reliably detected by visual inspection of the laid up structure which is costly and subject to error, especially when a portion of backing material is located in the layup where it can not be visually observed. Thus, there has been a continuing need for an automated system to detect backing materials inadvertently left in layups prior to curing.
The present invention has the technical advantages of providing a backing which can be detected by automated means, thereby greatly improving reliability and speed of inspection. Further, the present invention provides a quality control measure which can be used both prior to and after cure.